Q150 Transistor Ladder Filter (VCF)

The Q150 Transistor Ladder Filter is based on Bob Moog's famous design
and provides incredible-sounding -24dB lowpass filtering which has become
the standard of subtractive analog synthesis.

The cutoff frequency can be controlled manually and by control voltages
from an envelope generator, oscillator, keyboard, etc.
Frequency control signals can be attenuated and inverted without the need for utility modules.
Resonance (Q) of the filter can be adjusted with the front panel control.
The filter will self-oscillate and track a 1V/Octave keyboard control voltage.
Both -12dB and -24dB slopes are available offering a wide range of sounds.
One pole and three pole responses are jumper-selectable for even more possibilities.
A convenient two-channel mixer is provided for incoming audio signals.

A special feature of the Q150 is output leveling.
In most ladder filters the output signal level is greatly reduced as the resonance
is increased. This usually requires resetting input levels (sometimes many)
when changing resonance settings and can become quite annoying. Special circuitry in the
Q150 maintains constant output level at various resonance settings while keeping
the harmonic content unchanged.
This feature is jumper-selectable and can be deselected if identical behaviour to the Moog ladder filter is desired.

Basics
Most waveforms contain many different frequencies.
When an oscillator produces a sawtooth waveform, it can be thought of as a single
sine wave, and additional sine waves which are at multiples of the fundamental frequency and
are at lower amplitudes. These additional frequencies are called harmonics and different
waveforms have different amounts.
The Q150 Filter changes the way a waveform sounds by attenuating (lowering the amplitude)
of these harmonics.
This effect is especially useful when changing over time.
The frequency at which attenuation starts (or close enough) is called the cutoff frequency.
Cutoff frequency can be controlled manually or by voltage control.
Resonance (also known as Q, Regeneration, or Emphasis) has the affect of enhancing
frequencies near the cutoff frequency.

Frequency Control
The frequency control gives you about 10 octaves of cutoff frequency response.
The control inputs are added to the manual control's value to create the final cutoff frequency.
All of these signals work together at the same time to set the filter's cutoff frequency.

It's very common to have your filter track the keyboard so that the response
is the same over all frequencies.
This is accomplished by using the 1V/Octave frequency control input.
Simply patch your keyboard pitch voltage into a multiple
then out to your oscillators and to the filter.

The other frequency control inputs normally come from an Envelope
Generator or from an Oscillator.
You can attenuate, amplify or invert the incoming control signal right on the filter
instead of having to use another module.
The 0 to +5 volt outputs of the Q109 Envelope Generator
will give you a total of 10 octaves of range when the attenuator is full on.

The Sequential Controller can also be used to control
the filter frequency.
You could use one bank to control an oscillator and another to control the
filter at the same time.

Resonance Control
Resonance is the emphasis of frequencies near the cutoff frequency and has a
great affect on the sound.
The range of the resonance control is very large.
If you have too much resonance the filter might oscillate and clip.
We let you decide if this is good or not.
You can increase the maximum resonance possible by turning down the input signal level
control so that there is more room for resonance without clipping.

Self-Oscillation
Turning the resonance to maximum without an input signal will cause the filter to self-oscillate.
This allows the filter to act like an oscillator creating a very pure sine wave.

Signal Inputs
There are 2 signal inputs which are mixed together - both are adjustable.
Normally you will adjust the input levels to 50% or less to allow larger resonance peaks.

Outputs
The output is available at the bottom of the module.
A switch allows selection of -12dB or -24dB slopes.
Traditionally ladder filters have had -24dB slopes but we are also offering the -12dB slope to
give you even more sound possibilities.

A sine wave has almost no harmonics and will only respond to the filter by lowering its amplitude.
Sawtooth and Ramp waveforms have the most harmonics and respond quite nicely to filtering.
Square and Pulse waveforms also have a great deal of harmonics and respond well to filtering.

Noise and Hum
Filters are especially noisy and susceptible to interference and hum from the power supply.
Place the module as far away from the power supply as possible for lower hum.

A typical synthesizer patch will have a VCA (Q108) as the final module.
This has the effect of gating off noise in the modules up the chain including
the filter. If you use the filter as the final output without a VCA, much
more noise will be heard.

It's also important to remember that the outputs of a modular synthesizer
are designed to be very hot (10 volts peak-peak or more) in order to improve
the signal to noise ratio, so turn down the outboard equipment (mixers, etc)
and turn up the output levels of the synthesizer for best results.

MR Wrote: As a former Moog System 55 owner, I can say that the Q150 filter is as good as a 904a filter to my ears, it really is, and again before anyone wants to start a flame war I put them side by side... and actually Rogers was a bit "cleaner" too... so dare I say, almost better.